The invention relates to electronic infrared thermal sensors and to electronic circuits for detecting the outputs of such sensors.
Infrared thermal detectors have been and are used in a wide variety of applications which require room temperature operation and a uniform sensitivity over a wide spectral range. Among the most popular thermal detectors are the thermistor bolometer and the pyroelectric detector.
The thermistor bolometer is a thermal detector whose electrical resistance varies as a function of temperature. By measuring the resistance of the thermistor, its temperature can be deduced. In thermistors the electrical resistance usually decreases as the temperature of the thermistor increases.
The pyroelectric detector, on the other hand, is a thermal detector whose spontaneous polarization varies as a function of temperature. The spontaneous polarization, however, cannot be measured in equilibrium because it is exactly cancelled by the rearrangement of free charge in the material. Nevertheless, changes in the spontaneous polarization can be measured to detect changes in the temperature of the pyroelectric detector. Usually, with increasing temperature, the spontaneous polarization decreases.
Due to the different thermal effects on which operation of these two detectors is based, each detector has a different frequency response. The thermistor, in common with many other thermal detectors, is most sensitive at frequencies below the thermal relaxation frequency, .omega..sub.T, which is typically between 1 and 100 Hertz. Curve 10 in FIG. 1 is a plot of the logarithm of the responsivity of a typical thermistor bolometer versus the logarithm of the frequency of a sinusodially modulated incident radiation power to be detected. Curve 10 shows the responsivity remaining fairly constant up to the thermal relaxation frequency. Above this frequency the responsivity drops off quickly.
In contrast to the thermistor bolometer, pyroelectric detectors are most sensitive at frequencies above the thermal relaxation frequency. Curve 12 of FIG. 1 shows a plot of the logarithm of the responsivity of a typical pyroelectric detector versus the logarithm of the frequency of a sinusoidally modulated incident radiation power to be detected. Above the thermal relaxation frequency to a frequency, .omega..sub.e, which is determined by the electronic time constant of the circuit, the responsivity of the pyroelectric detector is relatively flat. At frequencies below the thermal relaxation frequency or above .omega..sub.e, the responsivity of the pyroelectric detector quickly drops off. It should be noted that in FIG. 1, although both curves 10 and 12 are shown on a single graph, the scales for each curve are not necessarily the same.
An example of a pyroelectric detector appears in U.S. Pat. No. 4,024,560 (Miller et al). Miller et al. disclose a pyroelectric field effect radiation detector. In FIGS. 1 and 2, for example, the pyroelectric material is electrically connected in series with a source of voltage (e.g. poling circuit 44) and with a load resistance (e.g. the resistance across the source and gate of the FET). Several possible materials are disclosed for the Miller et al pyroelectric detector, among which are triglycine sulfate (TGS), strontium-barium niobate (SBN), lithium niobate, and lithium tantalate.
Many materials are known which can be used as the sensing elements of thermistor bolometers, and many materials are known which can be used as pyroelectric detectors. Within these two classes of materials is a single subset of materials which exhibit both thermistor and pyroelectric properties. That is, the materials in this subset exhibit both changes in their electrical resistance with changes in temperature, and changes in their spontaneous electrical polarization with changes in temperature. Known pyroelectric materials whose electrical conductivity is strongly temperature dependent include single crystals of boracites, sodium nitrite (NaNO.sub.2), tin-hypothiodiphosphate (Sn.sub.2 P.sub.2 S.sub.6), lead germanate (Pb.sub.5 Ge.sub.3 O.sub.11), lithium ammonium sulfate (LiNH.sub.4 SO.sub.4), and some ferroelectric ceramics.